1. Field of the Invention
The present invention relates to an oscillator system such as a optical parametric oscillator or a laser. More specifically, the present invention relates to a tunable oscillator system which produces a single longitudinal mode output.
2. Description of Related Art
Tunable oscillator systems with relatively high output energies and narrow linewidths have a number of scientific and engineering applications. Narrow linewidth gas phase spectroscopy has become important in combustion research and atmospheric monitoring. Tunable oscillator systems which produce a single mode bandwidth over a broad tuning range are ideally suited for these applications.
Most tunable oscillator systems are implemented using lasers, such as dye lasers which are based on gain media dyes, each of which are tunable over a range of 15-100 nanometers. These systems are desirable because of the narrow linewidths and relatively high energy achieved by these systems. However, dye laser systems are cumbersome for applications requiring a wide tunable range because the dye gain medium must be changed as the output is tuned beyond the 15 to 100 nanometer range of the particular dye laser being used.
Optical parametric oscillators (OPO) and amplifiers represent another type of tunable oscillator system. OPO systems are tuned by rotating the angle of the optical gain medium relative to the optical path using servo techniques well known in the art. A variety of optical parametric gain media can be employed in OPOs, such as .beta.-barium borate (BBO), lithium tri-borate (LBO), cesium borate (CBO), and potassium titanyl-phosphate (KTP), each of which provide a different tunable range. OPOs are particularly interesting because of the wide range of tunability which they provide. For instance, tunable ranges from less than 400 nanometers to over about 2500 nanometers have been achieved by OPOs using .beta.-barium borate as the optical gain media.
High power single longitudinal mode outputs have been achieved by injection seeding a single mode seed source into a high power optical power source. The single mode seed sources used to produce a high power single longitudinal mode output are generally too weak to be used by themselves as a single mode output.
For example, Bosenberg, et al., (J. Opt. Soc. Am. B 10 1716-1722 (1993)) employs a tunable single frequency parametric frequency-conversion system in which single frequency pump radiation is obtained from the second harmonic radiation of a 10-Hz commercial Nd:YAG laser that is injection seeded, SLM and flash-lamp pumped. Then, by making the cavity of the seed source OPO very short (5 cm), Bosenberg, et al. produces a narrow OPO bandwidth of approximately 0.02 cm.sup.-1 which is used as a single longitudinal mode seed source. In order to tune the OPO system of Bosenberg, et al., it is necessary to carefully control the cavity length of the seed source, in addition to the crystal angle and tuning mirror, in order to maintain the production of a single longitudinal mode output over a broad tuning range.
Komine, et al., (Lasers '90: Proceedings of the 13th International Conference on Lasers and Applications. San Diego, Calif., Dec. 10-14, 1990, (STS Press, 1991) pp. 612-618) teaches an alternative approach for the production of an OPO bandwidth of less than 0.1 cm.sup.-1 for use as a single longitudinal mode seed source through the use of an intra-cavity etalon which serves to filter out unwanted modes. Etalons employ wavelength selective coatings which function as a spectral filter to isolate the desired single longitudinal mode output. Because the coatings used in etalons are wavelength selective, multiple optics are required to cover the tunable range of the oscillator system. Etalons also require angle tuning which adds an undesirable level of complexity to the production of a single longitudinal mode output, further encumbering one's ability to employ the single longitudinal mode output in a tunable oscillator system.
Spectral filters have also been used in the production of a single longitudinal mode output for use as a seed beam. For example, Fix, et al. (Digest on Conference of Lasers and Electrooptics '94 p.199-200) employs an extra-cavity etalon as a spectral filter in order to produce a single mode OPO output having a bandwidth of approximately 0.03 cm.sup.-1. As with the use of etalons, spectral filters, such as the grating used by Fix, et al., add an undesirable level of complexity to the production of a single longitudinal mode output which encumbers one's ability to employ the single longitudinal mode output in a tunable oscillator system.
The requirement that a single longitudinal mode seed source be used to produce a high energy single longitudinal mode output is disadvantageous in view of the complexity associated with producing a single longitudinal mode seed source. As discussed above, limiting the cavity length of the seed source as is taught by Bosenberg, et al. or using intra-cavity etalons, extra-cavity etalons or gratings to filter out a desired single mode seed beam, as is taught by Komine, et al. and Fix, et al., adds an undesirable level of complexity to the oscillator system which greatly limits the ability of these systems to scan over a broad range of wavelengths. It is therefore an object of the present invention to provide a tunable oscillator system which produces a high energy single longitudinal mode output from a multimode seed beam.